Just took another step toward what might become a real fusor power supply. Have long wanted 277 volt power for some nice new 60 mA NST's, bought cheaply on ebay 'cause of their primary voltage. Then last month, the 277 volt parking lot lights at work were changed from sodium to LED. Facility manager had the old luminaires stacked by the dumpsters. Pile gradually dwindled to nothing, but not before I took a couple home to play with. (Each unit is about 12 x 40 inches, with a 32 inch long lamp.)

Today's story might sound familiar on the weldingweb forum. My house has two 240V receptacles, one in the kitchen and one in garage for clothes dryer. The dryer has a now-deprecated 3 wire cord with NEMA 10-30 plug (hot-hot-neutral, no separate grounding conductor). But the wire run is entirely enclosed in steel conduits and boxes.

I spliced a spare dryer cord onto a long 10-AWG 4-conductor cord. The fourth wire is extended along the dryer cord and terminated with a ring lug, for a removable bond to the electrical box.

Time to verify that the steel enclosure could safely serve as an "intentional low resistance groundING conductor", with no local connection to the groundED conductor (white wire). Today's code doesn't permit my new copper water pipes to be used for grounding. Any wire connected to a water pipe is there to divert electricity out of the pipe, to protect plumbing users. Not to divert electricity into the pipe, to protect electricity users. And ground rod connections aren't sufficient -- in the event of a hot to ground short, the earth resistance is too high to keep voltage at a safe level, and to draw enough current to immediately trip the overcurrent protection device.

This called for a milliohmeter -- the opposite of a megger. With the dryer circuit turned off, the groundED and groundING wires at end of new cord were connected to a low-voltage, high-current transformer controlled by a variac.

It took 2.7 volts to get 15 amps. Outside of the picture, circuit runs through the neutral wire back to ground bar at the breaker panel, then returns through the steel conduit and cord wire #4.

The neutral wire resistance was much higher than expected. Then I found that it's only 12 AWG, unlike the black and red wires.
Still higher than expected. I found that the clip-on ammeter was indicating 15 A when actual current was 18 A.
Voltage drops posted above are based on 15.2 A measured with a current shunt resistor.

Next step: connect the big variac or a boost transformer, get 277 V, light up some sodium vapor.

Initial connection includes a 10 amp two pole circuit breaker / power switch. I was afraid it might trip from inrush current when variac was switched on, but no prob so far. Power-on indicator is a gratifying hum. With no load, the knob goes up to 290 V. No big wow -- clock radios in the UK have 240 volts in the cord. I adjusted it to 277 V.

Then lost little time connecting a parking lot luminaire, maintaining its designed orientation. It turned on uneventfully, starting with a deep red glow (neon fill gas?) and warming up to familiar yellow over a few minutes. Less than 1 amp from wallplug, if clip on meter is to be trusted on its 0-6 range. Parking lot used to have two lights on each of more than a dozen poles.

One next step is to improve the wiring enclosure. Covers, grommets & strain reliefs, crimped-on wire terminals, panel meters! Shopping list includes a new crimping tool -- apparently you really can squeeze too hard by hand, or I used the wrong slot for yellow insulated barrels on 10 AWG wire.

A simple boost transformer would be much quieter and more portable than the biggish variac. Or just feed the lamp units or NST's with straight 240 (which runs a bit high here). The lamp ballasts have two tap options: 277 and 208.

Right-o, Bob! Small buck or boost transformer stories to follow, but not before the real-world exercises.

Here's the ballast I got Friday night, from the very last luminaire. Its 43" long lamp was broken. In its place is a 29" "stubby" lamp previously salvaged from a different unit. Yellow glow hurts the eyes to look at, even in bright sunlight.

The U-shaped arc tube has little dimples in which sodium can condense. There's always condensed metal in there, even if molten -- I think Na is one of those metals with exceptionally low vapor pressure at its melting point. Along with Hg, whose vapor is also handy in electrical devices. Except of course when the tube breaks and fills with air!

Near the U-tube cathode ends, there's a progression of metal amounts in the dimples.

Hey Bob! An ad-hoc boost transformer, cascaded with an ad-hoc autotransformer,
has produced a tolerable example of 277 VAC at home with no variac, no 240 volt plug, and no semiconductors.

It being a problem that only a transformer nut would enjoy, it sure helps to have a transformer collection in the shed. This magnificent specimen has served from time to time as an isolation transformer (not 1:1, or even close) for experiments with saturation and fluxmetering in other transformers. The colored wire harness is left from whatever equipment it was pulled from. The thin green wire is an old test winding with a known turns count.

Couldn't have asked for a better ratio. Coloring outside the lines, terminals 3 and 5 are connected to the 120 V supply. 5 is also tied to 1. The 1-2 secondary voltage is almost perfect for boosting 120 V to 208 V, measured between 3 (common) and 2. No new soldering!

That's stage 1. Good enough for the sodium light units, if they were opened up & modified to use the 208 V pigtail wire instead of the 277 V wire.

But I had one extra coil-and-core unit from the luminaire graveyard. It was found there loose, after someone else had dropped it or dropped something on it. Secondary coil had a deep gouge, exposing severed copper windings. Not hard to disconnect and make sure it had no shorted turns.
Then the primary's Common, 208 V, and 277 V pigtails served as an autotransformer. Voltages measured with no load: Stage 1 input 120.9 V, interstage connection 207.9 V, stage 2 output 277.2 V. They didn't change much when the ballast secondary coil (the surviving 90% on long side of known break) was shorted.

It looks super clean in the picture, after being taken apart. Four welds cut with hacksaw, producing five blocks of laminated steel. Both coils were removed from the center block without any new damage to the windings. Doing that once in a lifetime is enough, and I wish I'd done it at 31 instead of 62. Details and application to follow.

This Fourth of July, the electromagnet sat idle while I worked on transformer stuff.
Finished a schematic of the work in previous post, in case the words & photos didn't get the message across.

A follow-on project is to convert that "ballast part" to get 208 and 277 volts directly from 120 volts, with 30 kV of isolation between the primary and secondary sides. The DIY approach is tedious, but semiconductor-free and, I hope, robust. Plan is to remove magnetic flux shunts from core assembly (to get rid of the intentional ballasting behavior) and put a 120 V primary coil in place of the existing secondary. Oil insulated, with things to learn about electric field strength.

ballast_section.PNG (5.28 KiB) Viewed 994 times

I think that can be done nicely, without having to wind or re-wind any magnet wire. The trick is to unwind some layers from the outside and inside of the factory-built secondary coil. Got most of the way, a few weeks ago. After each layer came off, the coils and core were reassembled for voltage ratio measurements. Easy, since the core laminations are assembled the cheap way (laminated blocks connected by welding, with no interleaving).

Today's work was to fill in a missing link: get to know the original ballast design. The magnetic part in hand was damaged to start with, but we also have a complete, working ballast circuit. See the 277 V sodium lamp demo in a previous post. Electrically, it looks like this:

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First round: Remove lamp, leave capacitor in place (16 uF 330 VAC). Energize "208 V" pigtail using a variac and 120:208 autotransformer; measure all voltages. Stop at the "nominal" point. Observed voltages with respect to Common:
v208 = 208
v277 = 278
vcap = 329
vlamp = 687. That's the open circuit voltage (RMS) available to start the lamp.
During the ramp up from 15% of nominal, primary side voltage ratios were constant. But at 15% and 30% of nominal, the secondary coil ratio was 3% and 1% lower than normal. Maybe that's a real nonlinear magnetization thing, as the flux divides beween the L-T core path and the shunt-with-airgap.

Second round: repeat without the capacitor. Vcap and vlamp were about 1% less than _with_ the capacitor. I guess the cap is there for power factor compensation, not for an intentional resonance at 60 Hz. (The caps used with ferroresonant transformers, for line voltage regulation, have roughly similar parameters.)

Third round: re-connect capacitor and install the lamp. Primary side ratios change slightly, I guess because of I*R drop, but we still got 325 V at vcap.
Vlamp was only 217 V to begin with (red neon glow phase). At the tube warmed up & brightened, the voltage crept upward.
A few minutes later it peaked at 240 V, with bright yellow glow.
Then started to come down. Ultimately stabilized at 166 V, with really bright yellow glow.
(The full length lamps produce 32,000 lumens at 176 lm/W, which is better than the LED replacements. But the LED's automatically dim to save energy, when nothing is moving in the parking lot. And the LED spectrum has a color rendering index much better than the "zero" assigned to L P sodium light.)

Fourth round: Determine the coupling between primary and secondary coils. Measure voltage ratio when primary is driven, then when "secondary" is driven. This time the power supply and voltmeter common point was connected to the CAP pigtail, common to both coils. Primary voltage was measured, and sometimes forced, at the COM pigtail. Secondary measurement and force point was the LAMP pigtail.
When driving whole primary coil with about 200 V, secondary coil voltage was 1.106 x primary.
When driving secondary with about 232 V, secondary voltage was about 1.210 x primary.
From those numbers, we get a turns ratio of 1.157 and a coupling ratio of 95.6%.
[edit] Similar measurements on the damaged unit had given a coupling ratio of 99.8%, after removal of the core shunts. Even with the L-T-L blocks merely stacked, not clamped together, so they were audibly growling.

Found time yesterday to set up a respectable load on "boosted 240 volt" power from the wall.
Marking new territory for me, the measurements went up to 326 volts at 7.9 amps (2575 watts).
Here are the details, in case anyone's interested.

The exercise used materials picked up at the flea market in July:
some 1000 watt 120 volt halogen lamps ($1 each), in home-made sockets

and a 315 VA control transformer with 230 volt input (part of a $8 bundle):

First step was to measure the magnetizing current of new transformer, to be sure it wasn't going to be too saturated at max output from the variac (nearly 290 V). Looked OK.

i_mag.PNG (6.22 KiB) Viewed 109 times

Then the transformer was connected to variac output in a boost configuration. Load is 3 lamps in series, and a 0.15 ohm current sense resistor.

boost.PNG (9.8 KiB) Viewed 109 times

All connections (except to voltmeters) are made with screw terminals or wire nuts.

Here's the measured current-voltage relationship. One run using 10 amp range on Fluke meter, until the probe leads began to smoke. Second run using measured voltage across the heat-sinked sense resistor. If left running long enough, the 10 amp breaker in primary circuit might trip.